1. Field of the Invention
The present invention generally relates to the technique of antennas. More specifically, the present invention relates to microstrip leaky-wave antennas utilized for wideband applications and its feeding system.
2. Description of the Prior Art
A leaky-wave antenna is generally utilized for high-frequency applications, especially for millimeter waves. Compared with traditional resonant antennas, the leaky-wave antenna has such advantages as higher manufacturing tolerance, simpler shaping and easier integration with feeding system, etc. In addition to the advantages mentioned above, because a leaky-wave antenna has a characteristic that the direction of the major lobe in the radiation pattern can vary in angle as the change of the operating frequency, it also can be utilized as a frequency-scanning antenna.
In general, there are two kinds of leaky-wave antennas for generating radiated waves. The first one utilizes periodic structure. That is, the energy in this kind of leaky-wave antenna is emitted by structural periodic disturbances that cause spacial harmonics, such as dielectric grating, metal plate grating, and slot grating on a metal slice. The second one utilizes open waveguides having the same shapes. Energy emission in this kind of leaky-wave antenna is achieved by the way in which the operation frequency of the propagation mode is assigned near to the cut-off region, such as groove waveguides, non-radiative dielectric waveguides and microstrips.
Because the microstrip line is manufactured by metal, its energy loss will much higher than that of leaky-wave antennas manufactured by high-Q(quality) dielectrics. In addition to being widely applied to various high-frequency applications, the microstrip leaky-wave antenna has various advantages, such as simple structures and easily manufacturing. Therefore, it is especially appropriate for the applications of integrated antennas and low-cost commercial antennas, etc.
FIG. 1 (PRIOR ART) is the perspective view of the conventional microstrip leaky-wave antenna. Microstrip leaky-wave antenna 10 is a strip of metal and placed at one side of dielectric material 20. The other side of dielectric material 20 is connected to a grounded metal plate 30. In addition, the width of microstrip leaky-wave antenna 10 is represented by W, the thickness of dielectric material 20 is represented by h, and the dielectric constant is represented by xcex5r. In general, dielectric constant is about larger than 2. The microstrip leaky-wave antenna should be operated around the cut-off region by utilizing the first higher order mode. Usually, the propagation way pertaining to the higher order modes in microstrips can be divided into four frequency regions as shown in FIG. 2 (PRIOR ART), which shows the relation between the normalized higher order mode phase constant (denoted by xcex2/Ko) and the normalized attenuation constant (denoted by xcex1/Ko) to the frequency (denoted by f). In FIG. 2, the phase constant of the higher order mode in the microstrip is represented by xcex2, the attenuation constant of the first higher order mode in the microstrip is represented by xcex1, and the wave number in air is represented by Ko. The curve of the normalized higher order mode phase constant xcex2/Ko and the curve of the normalized attenuation constant xcex1/Ko in FIG. 2 are represented by numerals 1 and 2, respectively. As shown in FIG. 2, there are four regions from high frequency to low frequency.
(I) Bound mode region
In this region, the normalized higher order mode phase constant xcex2/Ko is larger than 1 and the normalized attenuation constant xcex1/Ko is equal to 0. More specifically, the higher order mode phase constant xcex2 is larger than the phase constant of surface waves on the substrate (represented by xcex2s). That is, the energy in this region is bound in microstrip line and cannot be emitted.
(II) Surface wave region
In this region, the normalized higher order mode phase constant xcex2/Ko is between 1 and the normalized phase constant of surface waves on the substrate (i.e., xcex2s/Ko). A tiny amount of the attenuation constant is also apparent in this region. Due to the fact that the energy carried by the microstrip line leaks in the form of surface waves and cannot be emitted to the air, general antennas cannot utilize this region. Besides, the tiny amount of the attenuation constant represents the energy leakage in the form of surface waves.
(III) Space wave region
In this region, the normalized higher order mode phase constant xcex2/Ko is lower than 1. It means that the energy can be coupled to be the surface waves and the space waves. Due to the fact that most of the energy is coupled to the air, this region can be used to implement antennas. Besides, the attenuation constant in this region is larger than that in the surface wave region, which means the energy leakage of surface waves and space waves in physics.
(IV) Cut-off mode region
In this region, the attenuation constant is larger than the phase constant, which means that the cut-off feature can dominate the operation of the microstrip lines. Therefore, this region cannot be used in the applications of energy emission. Most of the fed signal energy will be reflected. Therefore, it is difficult to design appropriate antenna structures and the energy emission of such antennas is not efficient. Due to the reasons mentioned above, this region is not appropriate for antenna applications.
According to these kinds of microstrip higher order mode regions mentioned above, the microstrip leaky-wave antenna can be appropriately operated in the space wave region, more specifically, by using the first higher order mode operated near the cut-off region. The cut-off frequency of the higher order modes of the microstrip can be described in details as follows. The microstrip leaky-wave antenna is different to the closed waveguide. There is no obvious separation between neighboring operation regions like the closed waveguide due to the leaked energy near the cut-off region. In fact, the phase constant of the closed waveguide has an imaginary part (Y =jxcex2) in the higher frequencies at the separation point, which means that the wave can be propagated. In addition, there is a real number (Y =xcex1) of the propagation constant in the lower frequencies at the separation point, which means the attenuation of the propagated energy. On the contrary, there are no specific cut-off separation points for open microstrip higher order mode waves. For example, using the cavity model, the cut-off frequency of the microstrip leaky-wave antenna structure shown in FIG. 1 can be roughly defined as:                               f          c                =                  c                      2            ⁢            W            ⁢                                          ϵ                r                                                                        (        1        )            
Wherein the light speed is represented by c, the width of microstrip 10 is represented by w, and the relative permittivity of dielectric material 20 is represented by xcex5r. Next, the frequency bandwidth is described as follows. As described above, the space-wave mode is the most appropriate one for antenna applications and the normalized propagation constant is between 1 and the cut-off point. Using this relation, the radiation bandwidth can be deduced as:                               f          c                 less than         f         less than                                             f              c                        ⁢                                          ϵ                r                                                                                        ϵ                r                            -              1                                                          (        2        )            
As mentioned above, the dielectric constant of the substrate is usually larger than 2. The maximum usable bandwidth of the traditional microstrip leaky-wave antennas, according to the radiation bandwidth defined in equation (2), is about 40%. The usable bandwidth in practical applications usually cannot reach even 20% while considering other factors such as the bandwidth of the feeding system, the limitation of the antenna size (length) and the antenna gain etc.
Besides, the characteristic that the direction of major lobe will be varied in angle as the change of the operating frequency can be described by using the equation below. In other words, by the concept of whether phase angle is matched, the angle of the major lobe xcex8 of the antenna can be determined as the equation below:                     θ        =                              cos                          -              1                                ⁢                      β                          k              0                                                          (        3        )            
The phase constant xcex2 can change as the varying operating frequency. According to equation (3), the angle of the major lobe xcex8 also changes during using the antenna. These kinds of antennas can be utilized for applications of phase array antennas by utilizing the characteristic above. That is, one scanning dimension is controlled by utilizing conventional phase shifters and the other scanning dimension is controlled by utilizing the change of the operating frequency. Therefore, phase shifters originally used in the one-dimensional control mechanism for these phase array antennas can be waived. Utilizing the microstrip leaky-wave antenna to manufacture a phase array antenna is low-cost due to the reduction of the expensive phase shifters. On the other hand, high-gain antennas, or called the point-to-point satellite receiver antennas, can also be manufactured by utilizing the microstrip leaky-wave antenna. However, the shift of the main beam in this kind of antennas will cause a problem in their application. More specifically, if these antennas are applied to the narrow bandwidth applications, such as around 1% of the bandwidth, the shift of the main beam is quite small. However, if these antennas are applied to wide-band applications, such as larger than 10% of the bandwidth, the shift amount of the main beam is huge based on equation (3). It will cause such problems as disturbance or the degradation of the system quality for the point-to-point communication.
According to the reasons mentioned above, the microstrip leaky-wave antenna could be easily applied for some specific applications, but not appropriate for some other applications due to their characteristics. According to equation (2), for example, the bandwidth of the microstrip leaky-wave antenna is narrow, which makes it difficult to be applied for wideband applications.
In addition, the bandwidth of the feeding structure also must be large enough to achieve the optimal bandwidth of the antenna in practical applications. Otherwise, the inherent bandwidth of the antenna would be limited. Generally, the feeding structure utilizes the scheme of one-mode excitation to avoid the loss of coupled energy. FIG. 3 (PRIOR ART) shows the schematic view of a conventional microstrip leaky-wave antenna. In FIG. 3, numeral 100 represents the microstrip in the leaky mode, and numeral 110 represents the microstrip in the exciting bound mode. The exciting bound mode can be one of the microstrip line dominant mode, the slotline dominant mode and the conductor-backed coplanar strips dominant mode (abbreviated by CBCPS). In addition, there is a feeding transition 200 between microstrips 100 and 110, for transforming the modes at the both sides. FIG. 4 (PRIOR ART) shows the layout diagram of the microstrip leaky-wave antenna virtually utilized in the CBCPS feeding system. In addition, FIG. 4 also marks the parts corresponding to the components of the microstrip leaky-wave antenna shown in FIG. 3. Besides, one side of microstrip 110 is connected to a high frequency connector 130 (such as the SMA-type connector) utilized for feeding energy.
Therefore, an object of the present invention is to provide a wideband microstrip leaky-wave antenna, which has an increased operating bandwidth and can be utilized for various diverse communication applications.
Another object of the present invention is to provide an antenna feeding structure, which can be applied to wideband microstrip leaky-wave antennas, thereby preventing from unnecessarily limiting the antenna bandwidth.
The present invention achieves the above-indicated objects by providing a wideband microstrip leaky-wave antenna including a plurality of antenna sections. Each antenna section includes a microstrip line, which is utilized for leaking energy. The microstrip lines of these antenna sections have different and continuous widths, and these antenna sections are connected sequentially by the way in which the widths of the microstrip lines decrease. The band of these antenna sections depend on their corresponding widths, and these bands of the antenna sections constructs the bandwidth of the whole antenna.
In addition, such a wideband microstrip leaky-wave antenna can be connected to a feeding system that does not limit the bandwidth of the antenna. The feeding system comprises at least one pair of conductor-backed coplanar strips (CBCPS) connected to one end of the connected antenna sections, a balun connected to the CBCPS and an input microstrip line connected to the balun. The input microstrip line is utilized as energy input and transmits the received energy to the end of the connected antenna sections through the CBCPS and the balun. In addition, a metal bottom plate is located under the CBCPS and a dielectrics layer is located between the CBCPS and the metal bottom plate. The CBCPS represents a pair of microstrip lines for transmitting signals with equal amplitudes and the phase differences of 180xc2x0, or for coupling the odd modes in the microstrip lines.
In addition, the antenna feeding system can also be utilized for dual-major-lobe applications. That is, the antenna feeding system is utilized for connecting a first microstrip line and a second microstrip line, and transmitting energy to the first microstrip line and the second microstrip line. The first microstrip line and the second microstrip are wave-emission sources with symmetrical major lobe""s directions. The antenna feeding system comprises at least one first CBCPS connected to one end of the first microstrip line, at least one second CBCPS connected to one end of the second microstrip line, a balun for connecting the first CBCPS at one side and connecting the second CBCPS at the other opposite side, and an input microstrip line connected to the balun for transmitting energy to the first microstrip line and the second microstrip line through the first CBCPS, the second CBCPS and the balun.